Migration

Migration is defined as the regular, usually seasonal, movement of all or part of a population of animals. Many different animals migrate, including birds, hoofed animals, bats, whales, seals, and salmon. One-way movement of animals that do not return is called emigration. Emigration is due to different causes and is not considered migration. Also, the regular daily movements that many animals undertake are not considered migration.

Most birds and mammals that migrate follow an annual or seasonal pattern related to cyclic variations in temperature, vegetation, or precipitation. Salmon and some other fishes do not migrate annually. Instead, they return to their place of birth in order to reproduce. Some ethologists do not consider these movements migration but reserve the term for cyclic movements. In some parts of the world, animals will suddenly move into a new area temporarily. These sudden and temporary movements are called irruptions. Irregular movements, such as irruptions, are generally due to population growth during periods of abundant food followed by dispersal when food supplies diminish.

Most migrations involve horizontal movement. Animals move north and south with the seasons or move in a circular pattern to take advantage of cyclic rain patterns or new forage growth. Some animals, however, migrate by changing elevation. Aquatic animals may move from deeper water to the surface according to the season. Many birds, mammals, and insects migrate to higher or lower elevations in mountainous areas. This kind of migration produces the same kind of change in the environment as horizontal migration but involves only small horizontal displacements.

Invertebrate Migration

Some shrimp and crabs migrate for purposes of reproduction. Pregnant females move into shallow water to lay their eggs. The shallow water environment has fewer large predators, so the chances of the baby shrimp growing to maturity is increased. After they mature the shrimp move into deeper water to feed.

Probably the best known insect migrant is the monarch butterfly (Danaus plexippus ). In the summer, these insects move northward as far as Canada's Hudson Bay, where they breed and reproduce. Adults from the last generation of the year migrate southward in autumn through Oklahoma andTexas. Many move as far south as the mountains of central Mexico. After overwintering in the tall firs of these mountains, they follow spring northward, breeding and laying eggs on their preferred milkweed plants. Since a few of the returning butterflies are members of the first generation that developed from the overwintered insects, they can be considered true migrators. Most insects that arrive in the north, however, have been born in route. The longest distance recorded for the complete flight of an individual monarch butterfly is 3,010 kilometers (1,870 miles), truly a remarkable flight for this tiny insect.

Vertebrate Migration

The three categories of migratory fishes are oceanodromous, anadromous , and catadromous . Oceanodromous fish live and migrate entirely in the ocean. The many species of herring with different migration patterns are typical of oceanodromous fish. Anadromous fish live in the sea and migrate to freshwater to breed. Pacific salmon are typical. They hatch from eggs in mountain streams or lakes. The young feed and grow in the freshwater, then migrate to the sea after a year. Adult fish usually remain in the sea for two or three winters, where they grow to full size. Then they undergo dramatic physiological changes and migrate back to the stream where they were born. There the females lay eggs that are fertilized by the males. Then both sexes die. Some Atlantic salmon breed two or three times.

Catadromous fish reverse the behavior pattern of anadromous fish. Catadromous fish spend most of their lives in freshwater, then they migrate to the sea to breed. Eels of the genus Anguilla are the best known. Both European eels and North American eels spawn in an area of the Atlantic Ocean known as the Sargasso Sea. The larval forms of the fish are carried by the Gulf Stream to the shallow waters of the continental shelves. After about two years, when the larval eels are about 8 centimeters (3 inches) long, a metamorphosis occurs. The nearly transparent free-swimming larval eels are transformed into bottom-dwelling, dark-colored, cylindrical fish. Their migration upstream is spectacular, as the young fish gather by millions, forming a dense mass several miles long. In freshwater, the eels grow to full size. They live for several years in freshwater, then undergo a final metamorphosis before they swim back to the Sargasso Sea to spawn.

Terrestrial reptiles and amphibians do not migrate regularly enough to be significant, although some species migrate vertically. Reptiles and amphibians have evolved other strategies to deal with adverse environmental conditions, such as hibernation and estivation. Sea turtles are the exception. Most sea turtles migrate to beaches to lay their eggs. They then disperse back into the ocean. Green sea turtles (Chelonia mydas ) lay their eggs on the coast of Costa Rica in Central America and then disperse through the Gulf of Mexico and the West Indies.

The taxonomic class Aves includes most of the best known migrating species. Most species of birds require a large input of food to maintain their body temperature and other behaviors. Many birds have evolved behaviors that allow them to move to areas where food is more abundant. Birds have evolved a highly efficient means for traveling swiftly over long distances with great economy of energy.

Migratory birds do not differ greatly in gross physiological characteristics from nonmigratory birds. There is a spectrum of birds from completely nonmigratory species to species that fly thousands of kilometers every year.

Since insect populations drop dramatically during the winter, insectivorous species of birds, such as warblers, flycatchers, and wagtails, are highly migratory and typically spend the winter in the tropics. The geographical arrangement of the North American continent determines migration routes for many species of North American birds. Principal routes are known as flyways. They include the Mississippi flyway, the central flyway, the Pacific flyway, and the Pacific oceanic route. Many birds spend the winter in the states that border the Gulf of Mexico, but the principal wintering areas are in Mexico and Central America. Panama has the greatest density of winter bird residents in the world.

Tropical regions do not have the four seasons of temperate regions, but they do have cyclic rainy and dry seasons. Birds of tropical regions migrate according to these wet and dry seasons.

Arctic terns (Sterna paradisaea ) are the world champion migrators. These birds breed in the coastal regions of northern Europe, Asia, and North America. They then fly south and spend the winter in the extreme southern Pacific and Atlantic along Antarctic pack ice 17,600 kilometers (10,940 miles) from their breeding range. American populations of the Arctic tern cross the Atlantic to Europe, then fly south along the coast of western Europe. Arctic terns thus travel farther than any other bird species.

Most terrestrial mammals do not migrate. True migration among terrestrial mammals occurs mostly among large hoofed animals living in habitats with wide fluctuations of climatic and biotic conditions. For example, before the central United States was largely fenced in, American bison (Bison bison ) migrated regularly. Large herds containing millions of animals moved in circular routes to the southern part of their range in winter and back north when spring rains brought fresh grass to the northern part of their range.

In North American Arctic regions, caribou (Rangifer tarandus ) regularly migrate between the open tundra, where they calve, and the forest, where they spend the winter months. In winter, each caribou herd moves independently of the other herds in response to local conditions. Then in the spring, the herds move back onto the tundra. These migrations follow the same routes from year to year.

In contrast to terrestrial mammals, marine and flying mammals typically do migrate because of their inherently greater mobility. The only true flying mammals are the many species of bats. Huge colonies of Mexican freetailed bats (Tadarida brasiliensis ) spend the summer in Texas and adjoining states. The Congress Avenue Bridge in Austin, Texas, is home to the largest urban bat colony in the world, with around 1.5 million bats. It is a huge bat nursery containing females and nursing babies, called pups. Since they are insectivorous, these bats leave around the middle of November for their winter home in Mexico. They return in mid-March when the insect population in Texas increases dramatically. Most of the females in the Congress Avenue colony give birth to a single pup in early June.

Among mammals, the marine mammals are the distance record holders. Antarctic whales, such as humpback whales (Megaptera novaeangliae ), migrate regularly to the tropics. Whales migrate to areas rich in food, particularly the northwestern coast of Africa, the Gulf of Aden, and the Bay of Bengal. Northern whales, such as blue whales (Balaenoptera musculus ), have the same migratory habits as Antarctic whales. They migrate northward along the east coast of the United States, then through Davis Strait to Baffin Bay (north of Canada) or to waters off northern Scotland or the coast of Norway.

Migration Routes

Birds tend to follow well-defined migration routes called flyways. A population of birds may be scattered over thousands of square kilometers. As they begin their migration, the birds may be spread out over a migration front hundreds of kilometers wide. These routes are determined by geographical factors, ecological conditions, and meteorological conditions. Some routes cross oceans. American golden plover fly over open ocean from the Aleutian Islands southwest of Alaska to Hawaii, a distance of 3,300 kilometers (2,050 miles).

Many birds fly at relatively low altitudes. Hawks and other passerines, however, fly at altitudes as great as 4,000 meters (13,120 feet). The highest altitude ever recorded for migrating birds is 9,000 meters (29,520 feet) for geese near Dehra Dun in northwest India.

Some birds fly nonstop. Others are diurnal , flying during daylight hours and resting at night. Pelicans, storks, birds of prey, swifts, swallows, and finches migrate during daylight hours. Other birds reverse the pattern. Cuckoos, flycatchers, thrushes, warblers, orioles, and buntings fly at night and rest during the daylight hours.

Most birds abandon their instinctive territoriality during migration. Even unrelated birds with similar habits sometimes travel together. Some birds migrate in large flocks. Geese, ducks, pelicans, and cranes fly in well-known V-shaped formations that allow each bird to receive lift from the bird just in front.

Finding Their Way

Animals use several different techniques to navigate while migrating or to locate the place of their birth. Experiments have demonstrated the ability of animals to orient themselves geographically. Starlings have returned to their nests after being moved 800 kilometers (500 miles) away; swallows have found their way home from more than 1,800 kilometers (1,120 miles). A Manx shearwater (Puffinus puffinus ) flew 4,900 kilometers (3,040 miles) across the Atlantic from Massachusetts to Britain in twelve days. Laysan albatrosses (Diomedea immutabilis ) found their way back to Midway Island in the Pacific after being released at Whidbey Island, Washington. The journey covered 5,100 kilometers (3,170 miles) and took ten days. Experiments with fish and mammals have demonstrated similar homing ability.

Some homing animals use landmarks. The use of landmarks, however, cannot explain how migrants find their way along routes covering many hundreds or thousands of kilometers.

Birds apparently possess a compass sense. This sense is probably related to a sensitivity to Earth's magnetic field. When homing pigeons were released with tiny magnets attached to their necks, they were unable to navigate. Experiments have also shown that the orientation of birds is partly based on celestial bearings. In one well-known experiment, indigo buntings were placed in compartments in a planetarium and shown star patterns. Scratch marks on sensitive paper showed that the birds attempted to move "north" according to the star patterns displayed on the dome of the planetarium.

Experiments have shown that salmon and similar fish apparently use Sun orientation while at sea to find their way back to the general area of the stream in which they hatched. Once in the correct general area, salmon apparently use their sense of smell to locate their home waters.

What Triggers Migration?

Migration is part of the life cycle of animals. Metabolic patterns usually change prior to migration, and fats accumulate in the body tissues. Food consumption increases in the autumn reaching a peak at the beginning of the migration season. These fundamental physiological changes are apparently controlled indirectly by the pituitary gland. The pituitary acts as a sort of internal clock. Variations in temperature and hours of daylight are detected by the pituitary gland. The pituitary then influences the development of gonads and all other metabolic processes, including the thyroid gland. The thyroid gland excretes the chemical substances that cause the physiological changes that prepare the animal for migration. The pituitary serves only to prepare the animal for migration. Actual migration is triggered by appropriate environmental conditions, such as precipitation, availability of food, temperature, and weather conditions. A sudden period of cold weather during autumn may induce the immediate departure of many migrants.

Evolution and Ecology of Migration

Migration as a behavior among birds and mammals probably appeared gradually. Erratic dispersals were probably the precursors of true migration. Such erratic dispersal would have led to greater survival rates and reproductive success among animals that moved to the most favorable places. These originally erratic movements gradually acquired stability through natural selection . In some cases, original habitats were in presentday wintering areas, and animals developed a tendency to leave in spring in order to breed in other territories. Seasonal changes of weather and food supply in these newly settled regions forced the animals to migrate in fall, and they thus retreated to their former range. Many birds now nesting in the Northern Hemisphere, such as hummingbirds, tyrant flycatchers, tanagers, orioles, and swifts, have distinct tropical characteristics. These birds may have gradually spread northward as glacial ice receded.

The evolution of migration must be related to the ecological significance of migration. Migration allows fast-moving animals to exploit variations in resources and to move into areas where they could not remain year-round without the ability to move rapidly. Exploitation of peaks of food production, such as the dramatic increase of insect populations in temperate regions in the springtime, would not be possible without migratory populations.

see also Habitat.

Elliot Richmond

Bibliography

Aidley, David J., ed. Animal Migration. New York: Cambridge University Press, 1981.

Billington, Elizabeth T. Understanding Ecology. New York: F. Warne, 1971.

Curtis, Helena, and N. Sue Barnes. Biology, 5th ed. New York: Worth Publishing, 1989.

Miller, G. Tyler, Jr. Living in the Environment, 6th ed. Belmont, CA: Wadsworth, 1990.

Orr, Robert T. Animals in Migration. New York: Macmillan, 1970.

Purves, William K., and Gordon H. Orians. Life: The Science of Biology. Sunderland, MA: Sinauer, 1987.

Schmidt-Koenig, Klaus. Migration and Homing in Animals. New York: Springer-Verlag, 1975.

Swingland, Ian R., and Paul J. Greenwood, eds. The Ecology of Animal Movement. Oxfordshire, U.K.: Clarendon Press, 1983.

Terborgh, John. Where Have All the Birds Gone? Essays on the Biology and Conservation of Birds that Migrate to the American Tropics. Princeton, NJ: Princeton University Press, 1989.

Mexican free-tailed bats are among North America's most important animals, from both an ecological and agricultural standpoint. They consume phenomenal volumes of insects nightly, a large number of which are agricultural pests. Serious bat population decline has been observed, largely due to human activities (i.e., the destruction of old buildings or the use of pesticide) and vandalism of important roosting habitats